Television receivers are susceptible to signal interference from various sources. The interference may arise as electromagnetic radiation which interferes with the broadcast RF signal. Typical sources of such interference are automotive ignition systems and household motors such as are found in vacuum cleaners and hair dryers, etc. The interference is manifested as bright or dark spots appearing in the TV picture.
Television pictures are formed by a scanning technique in which the image is partitioned into a plurality of horizontal lines. The horizontal lines are successively transmitted to the receiver where they are successively reconstructed into the original image on the picture screen. The signal information from image line to image line is highly redundant for a large percentage of images. R. Dolby, U.S. Pat. No. 2,996,576 took advantage of the line-to-line signal redundancy to minimize the effect of TV signal disturbances in video recording/reproducing systems. Upon detection of video signal loss, signal from the preceding image line is delayed by one image line period and substituted for the lost signal to minimize picture disturbance. Detection of signal loss in such recording/reproducing systems is facilitated by the fact that the video signal is recorded as frequency modulation of a carrier. Thus video signal loss may be detected by detecting the loss of the FM carrier, i.e., by envelope detection of the FM signal. See for example U.S. Pat. No. 4,199,780, R. J. Taylor, and U.S. Pat. No. 4,203,134, T. J. Christopher, et al.
Relatively large numbers of defects in video recording media makes it imperative to include signal compensation systems in video recording/reproducing apparatus. On the other hand the infrequency of particularly objectionable interference in broadcast TV signals has not warranted inclusion of systems in TV receivers for eliminating interference. However, with the incorporation of charge transfer device delay lines in TV receivers to perform comb filtering of the luminance signal, it has become practical to consider correcting video interference due to noise impulses. The interference correction is performed by video signal substitution as in the recording/reproducing systems, however, the detection of noise or defects in the video signal in the TV receiver presents problems different from defect detection in the recording/reproducing systems.
Typically the interference or noise creating the picture disturbance is not of the type or amplitude to cause elimination of the broadcast signal carrier, thus one cannot rely upon envelope detection of the broadcast signal carrier to determine when defects are present in the signal. In addition the broadcast signal is an amplitude modulated carrier so that amplitude changes are generally not indicative of the presence of noise or signal defects. Considering the baseband video signal, the dynamic signal range is very large and thus impulse noise can have signal values from barely perceptible to a level that actually overloads the receiver circuits.
It should be appreciated that the wide dynamic signal range makes detection of small signal defects particularly difficult, for to do so, the reference level of the defect detection system must track the average amplitude of the signal. Thus prior TV receivers having facility for noise cancellation were relegated to detecting only large levels of noise that exceeded a threshold determined by the maximum video signal level.
The present inventor, recognized that impulse noise, i.e., noise signal which may supersede the video signal on all or a part of an image line, typically has a broad energy spectrum and causes interference across many TV channels. Energy from the impulse is therefore likely to be included in the sound component of a particular TV signal even though the sound component of the TV signal is broadcast on a carrier separate from the video signal component of the particular TV signal. The sound component of the TV signal is a frequency modulated waveform, thus amplitude detection of such a signal for the purpose of detecting the presence of relatively small values of impulse noise is effective.